Stabilised emulsions, electrostatically

As discussed above, the total energy-distance of separation curve for electrostatically stabilised shows a shallow minimum (secondary minimum) at a relatively long distance of separation between the droplets. However, by adding small amounts of electrolyte, such minima can be made sufficiently deep for weak flocculation to occur. The same applies to stericaUy stabihsed emulsions, which show only one minimum, but whose depth can be controlled by reducing the thickness of the adsorbed layer. This can be achieved by reducing the molecular weight of the stabiliser and/or the addition of a nonsolvent for the chains (e.g., an electrolyte). 

This can occur if the energy barrier is small or absent (for electrostatically stabilised emulsions) or when the stabilising chains reach poor solvency (for sterically stabilised emulsions, that is if />0.5). For convenience, the flocculation of electrostatically and sterically stabilised emulsions will be discussed separately. 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

As is well known, a lot of effects of surfactants, like damping of surface waves, the rate of thinning of liquid films, foaming and stabilisation of foams and emulsions, cannot just be described by a decrease in interfacial tension or by van der Waals and electrostatic interaction forces between two interfaces. The hydrodynamic shear stress at an interface covered by a surfactant adsorption layer is a typical example for the stimulation of an important surface effect. This effect, shown schematically in Fig. 3.9., is called the Marangoni effect. 

Being surface active (375), surfactants lower the interfacial tension between the water and monomer phases. The decrease in interfacial tension allows smaller droplets to be formed more easily during dispersion of the monomer in the water phase. In addition, the thermodynamic driving force for coalescence is lowered as a consequence of the reduction in interfacial tension. For this reason, and especially because of the electrostatic and steric stabilisation mechanisms, emulsions prepared with surfactants are more colloidally stable. 

Styrene-butadiene latices were prepared by emulsion-free polymerisation in the presence of N,N-diethylaminoethyl methacrylate to supply a positive charge and their colloidal behaviour and interaction with anionic pulp fibres investigated. It was found that the latices were positively charged and stabilised by electrostatic repulsion and deposited readily on anionic fibres suspended in water, forming a monolayer on the fibre surface. Dewatering and drying resulted in coalescence of the particles and fibres covered with a polymeric film, which improves the bonding between the fibres. 9 refs. CANADA... 

Emulsions Stabilised by Ionic Surfactants—van der Waals, Electrostatic and Surface Area Extension Contributions... 

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